ANFO mixtures are commonly used as explosives in mining and in other applications. These mixtures provide effective blasting results, particularly when low bulk density explosive grade ammonium nitrate (EGAN) prill is used. Such EGAN is manufactured to have a porous outer surface, which adsorbs sufficient fuel oil to provide a slightly negative oxygen balanced explosive; and a porous inner volume, that lowers the density and provides voids that act as “hot spots” during the detonation process.
High bulk density agricultural grade ammonium nitrate (AGAN) is also useable in ANFO. AGAN is manufactured without introducing external and internal porosity, and hence there are some technical problems that need to be overcome so as to enable its use in ANFO.
Other sources of ammonium nitrate are also known, which have been manufactured by a process similar to AGAN where the level of porosity is minimal, but which have a bulk density similar to EGAN because of the inclusion of a large dimple or hole.
The main technical disadvantages of ANFO are that (i) the product is damaged by the presence of relatively small amounts of water; (ii) the explosive energy of the mixture per unit volume (the bulk strength) is fixed for a given ammonium nitrate prill, dependant only on its bulk density; and (iii) the velocity of detonation (VOD) is limited to relatively moderate values. These disadvantages may be overcome by mixing ANFO with an ammonium nitrate based emulsion (ANE) in various proportions.
An ANE is a water-in-oil emulsion, where the dispersed water phase is comprised of ammonium nitrate, water, and other minor components, and the continuous oil phase is comprised of emulsifiers and carbonaceous liquids or solids. As ANEs are more expensive than ANFO, the blend ratio used in an explosive composition is generally the minimum needed to provide the required water resistance, bulk strength, VOD, or combination thereof.
Mixtures of ANE and ANFO that are comprised of 1% to 50% ANE and 99% to 50% ANFO are known as heavy ANFO (HANFO) mixtures. HANFO mixtures are used to provide a higher bulk strength product for use in ground which requires a higher level of energy to be effectively blasted; and at the higher levels (above 40% ANE) some water resistance. Mixtures with 50% to 100% ANE and 0% to 50% ANFO generally need to be sensitized by addition of chemical gassing agents or solid sensitization in order to detonate efficiently and are commonly referred to as “slurry” emulsion/ANFO blends. Such ANE and ANFO mixtures that are sensitized using chemical gassing agents are known as gassed blends. Emulsion/ANFO blends, including gassed blends, provide explosive compositions with a significant level of water resistance, and also allow a higher VOD to be obtained. These blends are used for charging into wet blastholes, sleeping the product in wet conditions, and for use in ground which is composed of rock with a higher compressive strength, and requires an explosive with a higher VOD (ie, more brissant) to blast it; or where a greater level of fragmentation of the ground is required.
It is generally preferred to use EGAN in HANFO and in gassed blends. However difficulties with the availability of the product, its cost, and its quality often mean that the use of the other ammonium nitrate types in explosive compositions will be attempted. There are some significant technical difficulties with this, arising from the lack of internal porosity of the prill to provide sensitization of the mixture; and the lack of external porosity to absorb the required level of fuel oil to provide the required slightly negative oxygen balance. In particular, if non-EGAN is used to manufacture ANFO, the fuel oil is not absorbed into the surface of the prill and wicking may occur resulting in diesel oil seeping away and into the ground, leaving behind the ammonium nitrate prill. The displacement of diesel will also change the explosive properties, resulting in an explosive with a positive oxygen balance and increased risk of post-blast fumes occurring. If such ANFO is mixed with emulsion to form a HANFO or gassed blend, the unabsorbed diesel will mix with and then dilute the emulsion. The viscosity of the emulsion will decrease and the product may not maintain its column integrity in the hole. The decreased viscosity emulsion can seep into cracks and fissures in the hole, causing slumping of the product.
The standard methods of overcoming these shortcomings are to: (i) use a mineral oil as the fuel component, which has a significantly higher viscosity than diesel fuel oil, which is retained by the AGAN at a higher level, and which leads to a smaller loss of viscosity in the emulsion phase should they be mixed; and (ii) for emulsion blends, replacing the diesel used on the ANFO by incorporating a higher level of fuel phase into the ANE. Using the later described method means that the viscosity of the emulsion is not compromised, and the increased fuel phase present accommodates for the non-incorporation of diesel in the ammonium nitrate component. But both of these methods are relatively expensive, in terms of the raw materials used or the change in production parameters required.
One way to address these issues is to ensure that the fuel oil is retained by the prill. By increasing the fuel oil absorption and adsorption capacity of the non-EGAN prill, wicking and dilution are avoided. It has been known to use additives with the fuel oil that aid in coupling the fuel oil to the surface of the AGAN prill. An example is described in Canadian Patent Application 2438161A1 which consists of epoxidized oils, vegetable oils, and ester derivatives of such being added to the fuel oil. Another example involves using solid fuel sources such as carbon black, as described in U.S. Pat. No. 3,540,953. However, using such materials requires the modification of existing explosive delivery machinery and it use can result in a build up of material that can clog key equipment. To avoid problems arising from such a build up, it would therefore be advantageous to have a binding agent that would dissolve in a fuel oil and would require no further modification to current explosive delivery equipment. It can also be chemically different than the known coupling agents described above to provide an alternative, and it would also be useful if it could have improved functionality, particularly in heavy ANFO type products and emulsion/ANFO blends. Another potential advantage is to utilise new components that have a new supply source, and which can be economically substituted for some of the oil previously used in these blasting formulations.
Accordingly, it would be useful to provide a new solution that avoids or ameliorates the disadvantages present in known approaches, or which provides an alternative to these approaches.